Drying of eggplant and selection of a suitable thin layer drying model

The thin layer drying behaviour of eggplants in a laboratory dryer was examined. Drying characteristics of eggplants were determined using heated ambient air at temperatures from 30 to 70 degreesC and velocities from 0.5 to 2.0 mls. Beside the effects of drying air temperature and velocity, the effects of pre-treatment and slice thickness on the drying characteristics, drying time and quality of dried product were also determined. Drying curves obtained from the experimental data were then fitted to the different semi-theoretical and/or empirical thin layer drying models. The effects of drying air temperature and velocity on the model constants and coefficients were evaluated by a multiple regression technique. All the models were compared according to three statistical parameter; i.e. root mean square error, chi-square and modeling efficiency. The results have shown that, increasing the drying air temperature and velocity causes shorter drying times. In an eggplant drying process, pre-treatment must definitely be applied to decrease drying time. A thinner slice also causes a shorter drying time. While the increasing drying air temperature decreased the colour lightness, increasing drying air velocity raised it. Slice thickness did not have any significant effect on the colour characteristics. Increasing drying air velocity raised the re-hydration ratio, and the highest value was obtained at a drying air temperature of 50 degreesC. The Midilli et al. model was found to be the best model for describing the drying curves of eggplants. The effects of drying air temperature and velocity on the drying constant and coefficient were also shown. (C) 2003 Elsevier Ltd. All rights reserved.